A 71-year-old male patient with a history of arterial hypertension, diabetes mellitus, dyslipidemia, COPD, pulmonary embolism, tachycardia–bradycardia syndrome treated with a dual-chamber pacemaker, and antiarrhythmic treatment (beta-blockers) is admitted to the Cardiology Department for an episode of decompensated heart failure. The patient presented aggravated dyspnea on exertion, paroxysmal nocturnal dyspnea, and important peripheral edema, which had progressively worsened during the previous 3–4 weeks. His ECG recorded at admittance to the Cardiology Department is presented in Fig. 1. The pacemaker was programmed in DDD mode, with a lower rate interval of 60 bpm and an upper tracking rate of 120 bpm.